1. Field of the Invention
This invention relates generally to a system and method for providing high voltage bus control in a fuel cell vehicle in the event that a high voltage battery fails and, more particularly, to a system and method for providing high voltage bus control in a fuel cell vehicle in the event that a high voltage battery fails, where the system employs a fuel cell boost circuit for coupling a fuel cell stack to the high voltage bus that adjusts a high voltage set-point based on the fuel cell stack voltage during the battery failure.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
Most fuel cell vehicles are hybrid vehicles that employ a rechargeable supplemental high voltage power source in addition to the fuel cell stack, such as a battery or an ultracapacitor. The high voltage power source provides supplemental power for the various vehicle auxiliary load, for system start-up and during high power demands when the fuel cell stack is unable to provide the desired power. More particularly, the fuel cell stack provides power to a traction motor and other vehicle systems through a DC voltage bus line for vehicle operation. The battery provides the supplemental power to the voltage bus line during those times when additional power is needed beyond what the stack can provide, such as during heavy acceleration. For example, the fuel cell stack may provide 70 kW of power. However, vehicle acceleration may require 100 kW or more of power. The fuel cell stack is used to recharge the battery at those times when the fuel cell stack is able to meet the system power demand. The generator power available from the traction motor can provide regenerative braking that can also be used to recharge the battery through the DC bus line.
In some fuel cell system designs that employ a high voltage battery, the high voltage components, including the electric traction motor, are electrically coupled to the high voltage bus. The high voltage bus is directly connected to the battery and operates off of the battery voltage, where a DC/DC fuel cell boost circuit is provided between the fuel cell stack and the high voltage bus to allow the fuel cell stack voltage to vary independently of the DC bus voltage. Alternately, the high voltage components of the system are electrically coupled to a high voltage bus that is directly coupled to the fuel cell stack so that the components operate off the stack voltage, where a DC/DC boost circuit is provided between the high voltage bus and the battery to allow the battery voltage to vary independently of the bus voltage.
In the design where the loads are coupled directly to the battery voltage, the loads are controlled so that they draw a voltage that is within the allowable voltage range of the battery, such as 300 to 400 volts. A supervisory controller is employed that knows the allowable voltage range of the battery and controls the amount of power that the various components can draw from the bus or provide to the bus during regenerative operation. The sum of all power flows equals the power discharged from or charged into the battery and is controlled, such as by the supervisory controller, so that it does not drive the battery voltage above its upper or below its lower voltage limit, respectively. However, because of measurement errors, controller area network (CAN) message transmission time, power transients, etc., the supervisory controller may allow the loads to draw/provide power levels that in total lead to violation of the battery voltage limits. Therefore, some loads feature voltage limiting functions based on an embedded algorithm that prevents the loads from drawing/providing more power from/to the bus than possible without violation of battery voltage limits. In other words, as long as the voltage control algorithms in the component know the upper and lower battery voltage limits, they can at least temporarily react to attempts to draw/provide too much power from/to the high voltage bus without input from the supervisory controller.
In the event that the battery fails, the battery contactors are opened to disconnect the battery from the bus, but the various high voltage components, such as the electric traction motor, can still receive power from the high voltage bus as generated by the fuel cell stack through the fuel cell DC boost circuit. Usually the fuel cell DC boost controls the high voltage bus to a fixed voltage level as the bus voltage is now not defined by the battery voltage anymore. For a particular system operating condition, the supervisory controller will set the cathode compressor, anode injectors, etc., for a particular maximum stack output current and cause the high voltage components to draw a total power level that matches this stack output current and the resulting stack output voltage.
However, for the reasons discussed above as a result of supervisory controller message lag time, voltage measurement inaccuracies, power transients, etc., the amount of power being drawn from the high voltage bus by the loads at any given time may exceed the stack power at maximum stack output current. If the loads draw power from the bus that exceeds the stack limit, the fuel cell boost circuit will draw more current from the fuel cell stack than the stack is currently able to produce, which will change the stack stoichiometries. This change in stack stoichiometry affects stack operating conditions, such as a desired stack relative humidity, which causes stack degradation. The supervisory controller will correct the media flow for the proper stack stoichiometry that the loads are attempting to draw from the stack, but this control may not be quick enough to prevent stack degradation at least for some period of time.